1. Field of the Invention
This invention relates to optical waveguide fibers and, in particular, to a method for monitoring fiber tension during the drawing of such fibers.
2. Description of the Prior Art
In the manufacture of glass optical fiber, the fiber is pulled from the end of a glass preform or draw blank which has been heated to a sufficiently high temperature. One of the more important process parameters in the production of optical waveguide fibers is the tension within the fiber during the drawing process, and, in particular, the tension within the fiber in the region between the hot zone and the first coater. The magnitude of this tension affects the final properties of the fiber, including the fiber's diameter, ultimate strength and, through a stress optic effect, its optical properties.
From a process point of view, the tension in the fiber also affects the overall stability and throughput of the drawing process. Excessive tension leads to rapid necking and ultimate rupture of the fiber in the hot zone region. If not carefully controlled, increasing the temperature of the hot zone to reduce fiber tension can result in draw resonance and root oscillation, which in turn can result in oscillatory variations in fiber diameter which are difficult to control with conventional fiber diameter monitoring equipment. Oscillation of the fiber during drawing can also adversely affect the fiber coating process.
Fiber tension is related to the viscosity of the glass in the root portion of the preform from which the fiber is being drawn, and to the speed with which the fiber is being drawn. Since the viscosity of the glass is a function of temperature, draw tension can be controlled by adjusting the temperature of the furnace.
The temperature of the furnace itself can be measured and controlled using conventional temperature detection techniques such as pyrometers or thermocouples. However, due to the thermodynamics of the draw process, this control does not provide adequate control of the root temperature. The part of the preform above the root acts as a heat sink which reduces the temperature of the root. If the furnace temperature remains constant, the root becomes hotter with a decrease in the size of the preform. A constant temperature furnace will therefore result in a lowering of the draw tension as the preform is reduced in size during the fiber draw process.
Draw tension can be controlled by measuring the draw tension at various times during the draw process, and then modifying furnace temperature to compensate for a reduction in preform length. As the preform size is reduced and the heat sink becomes smaller, the furnace temperature is lowered.
Fiber tension has been monitored mechanically by measuring the deformation of the fiber in response to a force applied transversely to the direction of motion of the fiber. U.K. Patent Application GB 2,179,339A discloses a three wheel device wherein two wheels are applied to one side of the fiber and a third wheel is applied to the other side of the fiber. The location of the third wheel relative to the first two wheels is used as a measure of the tension in the fiber. Application GB 2,179,339A teaches that the measurement is made below the coater, and that the signal from that tensiometer is used to control the temperature of the draw furnace only during the initial set-up procedure when no coating is being applied to the fiber.
The three wheel approach has numerous disadvantages. It is difficult to precisely align the device with the fiber so as not to change the original path of the fiber. Contact of the three wheel device with the fiber affects the on-line fiber diameter feedback loop so as to reduce fiber draw speed. Also, the moving fiber can break when contacted by the three wheel device. A tensiometer is preferably mounted just below the furnace when a coated fiber is being drawn. A break in this location results in lost production since it necessitates the re-starting of the complete draw process.
U.S. Pat. No. 4,692,615 discloses a non-contact method and apparatus for measuring tension in a moving fiber by sensing the motion of the fiber; analyzing that motion to determine at least one of its frequency components; and monitoring the frequency component or components so determined so as to monitor the tension in the fiber. That method is based on the fact that, at least to a first approximation, the vibrational behavior of an optical fiber during drawing corresponds to the vibrational behavior of a string under tension which has been fixed at both ends. The fiber forms a stretched string between the root and the first coating applicator. The wave equation of the stretched string is: EQU F=.mu.(2v.delta.).sup.2 Eq. 1
where F is the force on the fiber, .mu. is the linear density of the, v is the principal harmonic frequency, and .delta. is the suspended length of the fiber. If the principal harmonic frequency of vibration is measured, then the force, or tension can be calculated. Occasionally, the frequency peak of maximum magnitude results from noise caused by rotating machinery or other periodic vibration sources rather than the fundamental fiber vibration frequency.